1. Field of the Invention
Embodiments of the present invention generally relate to methods and apparatus for connecting threaded members while ensuring that a proper connection is made.
2. Description of the Related Art
When joining lengths of tubing (i.e., production tubing, casing, drill pipe, etc.; collectively referred to herein as tubing) for oil wells, the nature of the connection between the lengths of tubing is critical. It is conventional to form such lengths of tubing to standards prescribed by the American Petroleum Institute (API). Each length of tubing has an internal threading at one end and an external threading at another end. The externally-threaded end of one length of tubing is adapted to engage in the internally-threaded end of another length of tubing. API type connections between lengths of such tubing rely on thread interference and the interposition of a thread compound to provide a seal.
For some oil well tubing, such API type connections are not sufficiently secure or leakproof. In particular, as the petroleum industry has drilled deeper into the earth during exploration and production, increasing pressures have been encountered. In such environments, where API type connections are not suitable, it is conventional to utilize so-called “premium grade” tubing which is manufactured to at least API standards but in which a metal-to-metal sealing area is provided between the lengths. In this case, the lengths of tubing each have tapered surfaces which engage one another to form the metal-to-metal sealing area. Engagement of the tapered surfaces is referred to as the “shoulder” position/condition.
Whether the threaded pipe members are of the API type or are premium grade connections, methods are needed to ensure a good connection. One method involves the connection of two co-operating threaded pipe sections, rotating the pipe sections relative to one another by means of a power tong, measuring the torque applied to rotate one section relative to the other and the number of rotations or turns which one section makes relative to the other. Signals indicative of the torque and turns are fed to a controller which ascertains whether the measured torque and turns fall within a predetermined range of torque and turns which are known to produce a good connection. Upon reaching a torque-turn value within a prescribed minimum and maximum (referred to as a dump value), the torque applied by the power tong is terminated. An output signal, e.g. an audible signal, is then operated to indicate whether the connection is a good or a bad connection.
As indicated above, a leakproof metal-to-metal seal is to be achieved, and in order for the seal to be effective, the amount of torque applied to effect the shoulder condition and the metal-to-metal seal is critical. In the case of premium grade connections, the manufacturers of the premium grade tubing publish torque values required for correct makeup utilizing a particular tubing. Such published values may be based on minimum, optimum and maximum torque values, minimum and maximum torque values, or an optimum torque value only. Current practice is to makeup the connection to within a predetermined torque range while plotting the applied torque vs. rotation or time, and then make a visual inspection and determination of the quality of the makeup. However, in addition to being highly subjective, such an approach fails to take into consideration other factors which can result in final torque values indicating a good final make-up condition when, in fact, a leakproof seal may not necessarily have been achieved. Such other factors include, for example, the coefficient of friction of the lubricant, cleanliness of the connection surfaces, surface finish of the connection parts, manufacturing tolerances, etc. In general, the most significant factor is the coefficient of friction of the lubricant which will vary with ambient temperature and change during connection make-up as the various components of the lubricant break down under increasing bearing pressure. Eventually, the coefficient of friction tends to that of steel, whereupon the connection will be damaged with continued rotation.
Therefore, there is a need for methods and apparatus for connecting threaded members while ensuring that a proper connection is made, particularly for premium grade connections.